1. Field of the Invention
The present invention relates to a scanning exposure type reproducing apparatus such as a reproducing camera, an electrophotographic copying machine and the like, and more specifically to a synchronizing apparatus, that is, a synchronous driving apparatus for moving either an original holding frame or an optical system in synchronization with a recording sheet.
2. Description of the Prior Art
A synchronous driving apparatus of particular interest to the present invention is disclosed in, for example, Japanese Utility Model Laying-Open Gazette No. 81648/1986. FIG. 1 is a cross-sectional view showing the synchronous driving apparatus disclosed therein and FIG. 2 is a perspective view showing the main portion thereof.
Referring to FIGS. 1 and 2, a conventional synchronous driving apparatus comprises an endless steel belt 52 which is fixed to an original holding frame 51 and which moves according to the movement of the frame. The endless belt 52 is wound around a driving pulley 53. The driving pulley 53 drives a driving roller 54 which is provided coaxially with the driving pulley 53. The driving roller 54 rotates a feeding roller 56 and the feeding roller 56 feeds an image recording sheet 55. A pressing roller 57 presses the image recording sheet 55 onto the feeding roller 56.
The conventional apparatus forms an image on the image recording sheet 55 in synchronization with the scanning of the original. Thus if scanning and movement of the image recording sheet 55 are not carried out at the same speed, the image on the sheet 55 will be skewed. In order to prevent this problem, the conventional apparatus is carefully structured as described in the following. The driving pulley 53 and the driving roller 54 are made of materials which have the same expansion coefficient. The pulley 53 and the roller 54 have the same diameter. Therefore, even if there is a change in temperature, the diameters of the driving pulley 53 and the driving roller 54 will be the same, and their speeds of rotation will be the same. Consequently, there will be no difference between the speed of the original holding frame 51 and the speed of the image recording sheet 55, which would be otherwise caused by thermal expansion derived from a change in temperature.
In the conventional synchronous driving apparatus, synchronization between the rotation of the feeding roller and the movement of the original holding frame can be maintained even if there is a change in temperature. However, even when such synchronization is maintained, the quality of the reproduced and recorded image is sometimes degraded. The reason is as follows. Since the feeding pressing roller 57 (which is formed of rubber) presses the image recording sheet 55 on to the feeding roller 56, the speed of the feeding roller 56 and the image recording sheet 55 is theoretically the same. However, in practice when there is a change in temperature, the hardness of the rubber forming the feeding roller 56 varies, and the volume thereof expands or contracts. Consequently, the diameter of the feeding roller 56 changes. This causes a change in the coefficient of friction between the roller 57 and the image recording sheet 55. Therefore, the friction between the image recording sheet 55 and the feeding roller 56 changes as a function of temperature. Consequently, even if the driving pulley 53 is rotated at a constant speed, the image recording sheet 55 is not necessarily moved at a constant speed.